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Genetics: Heredity, Traits & Chromosomes

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  • 0:02 Genetic Selection
  • 0:44 Genotype and Phenotype
  • 2:32 Heritable Traits
  • 3:09 Chromosomes and the Genome
  • 4:53 Genetic Differences
  • 5:50 Lesson Summary
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Lesson Transcript
Instructor: Greg Chin
We view manipulation of genes in our crops and livestock as a recent development. Yet, man has been manipulating the genetic makeup of his food for thousands of years through cultivation and breeding. This lesson will begin to help you understand how genetics works.

Genetic Selection

The principles of genetics have existed for thousands of years - we just never called it 'genetics.' Think about farmers trying to cultivate heartier, more delicious crops, or horse breeders who want to develop faster, better racing horses. They would do this by only selecting the best horses in each generation for breeding. Without realizing it, early farmers and breeders practiced genetic selection before we really knew what genetics was. Thousands of years later, Gregor Mendel's studies of pea plants formed the foundation upon which modern genetics was built. But rather than read about pea plants though, let's go visit our modern-day scientist, Adrian, to see these principles in action.

Genotype and Phenotype

Adrian is excited because he's just discovered a brand new species of hamster that can fly. He thinks this is going to be a scientific gold mine so he decides to drop everything that he's been working on and study this new species.

Let's take a minute to talk about terminology so we can understand Adrian's work.

Genetic mutations in the DNA code cause changes in phenotype
Genetic changes

Remember that a gene is just a piece of DNA that encodes a gene product. So we can also refer to this DNA code as genotype, but it'll be difficult to track the genetic changes occurring in our flying hamsters. It'd be a lot easier to monitor observable traits.

Recall that the gene products are what provide functionality in the cells and for the organism. So in performing these various functions, gene products are producing observable traits. And these observable traits are referred to as phenotype. So what this means then is if I was to make a change in the genotype - let's say we change this 'T' and mutate it to an 'A' - a change in the genotype will in turn cause a change in the phenotype, which then can be observed by us as scientists.

Geneticists are interested in heritable traits. A mutation in the DNA can be passed on to offspring and in that way, this change in the DNA code has become a permanent change in the genome of this organism. However, not all traits are heritable.

Heritable Traits

For instance, if one of our experimental hamsters happens to break a leg, that hamster won't pass on a broken leg to its offspring; this is not a 'heritable trait'. Because of the link between genotype and phenotype, you can see how farmers would select for genetic improvements in their crops and their livestock by selecting for phenotypic changes occurring over time. The different types of corn that we see in the supermarket today - yellow, blue, white , etc. - are good examples of genetic manipulation by cultivation.

Chromosomes and the Genome

A diploid has two versions of each chromosome called homologs or homologous chromosomes
Homologous chromosome

So I think we have genotype and phenotype pretty square, but we need a few more terms to be able to really understand the full extent of genotype.

Now, genes are located on chromosomes, and this makes sense because we just got through saying how a gene is made up of DNA. A chromosome is just a discrete structure of DNA that's found in the genome.

By observing the chromosomes under a microscope, Adrian determines that the flying hamster, like most animals, is a diploid, and furthermore, he determines that flying hamsters have a total of six chromosomes.

So hopefully all those words ring a bell from our previous lessons, but here's another way of looking at it:

A chromosome is defined by a specific sequence of DNA, which in turn means it's defined by a specific set of genes; however, because the organism is diploid, it has two version of each chromosome. Each of these versions is called a homologous chromosome or a homolog.

So to summarize, our flying hamsters have three different chromosomes - we have chromosome one here, chromosome two and chromosome three. Because the flying hamster is a diploid, that totals up to six pairs of homologous chromosomes in each hamster cell.

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